High tensile strength steel and marine structure having excellent weld toughness

ABSTRACT

In order to provide a high tensile strength steel having excellent low temperature toughness and which can withstand large heat input welding, a steel comprises, in mass percent, C: 0.01-0.10%, Si: at most 0.5%, Mn: 0.8-1.8%, P: at most 0.020%, S: at most 0.01%, Cu: 0.8-1.5%, Ni: 0.2-1.5%, Al: 0.001-0.05%, N: 0.0030-0.0080%, O: 0.0005-0.0035%, if necessary at least one of Ti: 0.005-0.03%, Nb: 0.003-0.03%, and Mo: 0.1-0.8%, and a remainder of Fe and impurities, and the N/Al ratio is 0.3-3.0.

This is a continuation in part of application Ser. No. 11/443,849 filed on May 26, 2006, which is a continuation of International Patent Application No. PCT/W2004/017575, filed Nov. 26, 2004. The PCT application was not in English as published under PCT Article 21(2).

TECHNICAL FIELD

The present invention relates to a high tensile strength steel and a marine structure, and particularly to a high tensile strength steel for welding and a marine structure having excellent weld toughness.

More specifically, the present invention relates to a high tensile strength steel for welding for use in welded structures such as buildings, civil engineering projects, construction equipment, ships, pipes, tanks, and marine structures, and particularly to a high tensile strength steel for welding to construct marine structures as well as to a marine structure thus constructed. For example, it relates to a thick, high strength steel plate with a thickness of at least 50 mm and a yield strength of at least 420 N/mm² and to a marine structure using such a plate.

BACKGROUND ART

In recent years, as energy demands show a tendency to increase more and more, the search for undersea oil resources is being actively carried out. Marine structures used for this purpose such as platforms and jackup rigs are becoming large in size, and at the same time the thickness of steel materials such as steel plates used in such structures is increasing, so attaining higher safety has become an important issue.

In usual marine structures, medium strength steel having a yield stress on the order of 300-360 MPa has been used, but in large structures like those described above, extremely thick high tensile strength steel having a high strength on the order of 460-700 MPa and a plate thickness exceeding 100 mm is sometimes used.

The regions where searches for undersea oil resources are conducted have in recent years shifted towards cold regions and deep water regions. Marine structures which operate in such land regions or sea regions are exposed to extremely severe weather and ocean conditions.

Therefore, steel used in such marine structures is required to have toughness in extremely severe low temperature ranges such as at −40° C. or below. Weldability is of course also required.

From the standpoint of safety, the inspection standards of users have become more severe, and not only is the conventional Charpy impact value prescribed for base metals and welds, but the CTOD (crack tip opening displacement) value at the lowest temperature of use has also come to be prescribed to evaluate toughness. Namely, even when stable properties are obtained in a Charpy test which is an evaluation test performed on a minute test piece cut into a size of 10 mm×10 mm, there are many cases in which the CTOD properties, which are evaluated on a test piece having the actual thickness of a structure, cannot satisfy required properties. Today, even stricter CTOD properties are being demanded.

Thus, there is a strong demand for an increase in low temperature toughness of weld heat affected zones (referred to below as HAZ) not only for steels used in marine structures installed in icy waters but also for steels used in line pipe for use in cold regions having a milder environment as well as for large welded structures such as ships and LNG tanks.

In order to obtain a high toughness in a low temperature range of −40° C. or below, it is necessary to perform welding under low heat input welding conditions, which have poor welding efficiency. Welding costs represent a large portion of the construction cost of marine structures. The most direct method for decreasing welding costs is to employ a high performance welding method which can perform welding with a high heat input so as to decrease the number of welding passes.

Accordingly, today, in structures intended for cold regions where there are severe demands for low temperature toughness, it is important to carry out welding so that welding costs are as low as possible while taking into consideration the toughness of HAZ.

From in the past, it has been known that decreasing the level of C is effective in order to dramatically improve the toughness of a HAZ of a steel. In order to compensate for a decrease in strength due to the decrease in C, it has been attempted to increase the strength by adding various alloying elements or by utilizing the effect of age precipitation hardening. For example, ASTM A710 discloses a steel which utilizes the age precipitation hardening effect of Cu, and there have been a number of reports based on this concept.

For example, JP H07-81164 B, JP H05-186820 A, and JP H05-179344 A propose a Cu-precipitated steel having improved weld toughness.

However, JP H07-81164 B merely evaluates the Charpy properties of a welded joint obtained from a plate with a thickness of 30 mm using a welding heat input of 40 kJ/cm, and such steel cannot be truly considered one which can cope with high heat input welding.

JP H05-186820 A proposes a high tensile strength steel having a tensile strength of at least 686 MPa to which 0.5%-4.0% Cu is added. Regarding the low temperature toughness of this steel, however, in view of its transition temperature in a Charpy test which is only 30° C., the steel cannot really be considered to have low temperature CTOD properties in extremely thick steel plates.

JP H05-179344 A proposes a Cu-precipitated steel having improved Charpy toughness in welds. However, it merely evaluates Charpy properties of a welded joint obtained with a weld heat input of 5 kJ/mm, and it cannot be truly viewed as a technique for fully satisfying the safety of a structure at the time of high heat input welding.

DISCLOSURE OF THE INVENTION

Accordingly, the object of the present invention is to provide a high tensile strength steel for welding which is improved generally in low temperature toughness of welds and particularly in low temperature toughness in HAZ.

As a result of various experiments concerning a steel composition and a method for its manufacture with the object of developing a thick, high strength steel plate having excellent weld toughness, the present inventors made the following findings.

(i) Using Cu-containing steel as a base, the contents of N and Al are adjusted with controlling the N/Al ratio.

In a steel having a high Cu content, in order to improve the toughness of a HAZ with high heat input, it is effective to finely disperse carbides/nitrides such as TiN, Ti(C,N), and MN. As a result of studying steels with a high content of Cu and Ti, it was found that it is effective to adjust the N and Al content with controlling the N/Al ratio. This is thought to be because when the N/Al ratio is too small, coarse AlN precipitates, and not only does this itself have an adverse effect on toughness, but also it impedes fine dispersion of TiN in large amounts. On the other hand, if the N/Al ratio is too large, the amount of solid solution N increases, and the density of AlN and TiN dispersed in the steel becomes very small.

(ii) In order to increase yield strength, it is necessary to disperse finely precipitated Cu particles as uniformly as possible.

(iii) In order to improved toughness and particularly low temperature CTOD properties, it is necessary to coarsen the Cu particles to a certain extent and to suppress the amount of the dispersed Cu particles.

(iv) In order to uniformly disperse the Cu particles, the formation of Cu particles in any stage before aging is suppressed as much as possible, and the state of dispersion of Cu particles is controlled by controlling the conditions of aging.

(v) Concerning the distribution of Cu particles, by taking as factors the average value of the equivalent circle diameter of the Cu particles and the plane-converted area fraction occupied by the Cu particles which are both determined on a TEM photograph, it is possible to control the balance of strength and toughness.

(vi) Cu particles readily form on crystal defects (primarily on dislocations) in steel, and if the density of dislocations is high, precipitation of Cu particles is promoted. Cu particles precipitated on dislocations impede the movement of dislocations and increase yield strength.

(vii) The density of dislocations in steel can be controlled by the rolling and water cooling conditions. A decrease in the rolling temperature, an increase in the overall rolling reduction, an increase in the temperature at the start of water cooling, an increase in the cooling rate, and a decrease in the temperature at the completion of water cooling each increase the density of dislocations.

(viii) Using a high-Cu steel composition as a base, it is possible to stabilize the toughness of HAZ of welds formed with high heat input by controlling the hardenability by adjusting the contents of C, Mn, and Mo in the steel.

Namely, in a high-Cu steel, the more the weld cracking parameter Pcm decreases, the more HAZ toughness can be improved. It was found that a decrease in C and Mn is effective for this purpose. However, in order to achieve high strength, addition of other element is necessary. It was also found that addition of Mo in a controlled amount makes it possible to stabilize the balance between strength and toughness.

The present invention is based on such findings, and in its essence it is as follows.

(1) A high tensile strength steel comprising, in mass percent, C: 0.01-0.10%, Si: at most 0.5%, Mn: 0.8-1.8%, P: at most 0.020%, S: at most 0.01%, Cu: 0.8-1.5%, Ni: 0.2-1.5%, Al: 0.001-0.05%, N: 0.0030-0.0080%, O: 0.0005-0.0035%, and a reminder of Fe and impurities, wherein the N/Al ratio is 0.3-3.0.

(2) A high tensile strength steel as described above in (1) which further contains, in mass percent, Ti: 0.005-0.03%.

(3) A high tensile strength steel as described above in (1) or (2) further containing, in mass percent, Nb: 0.003-0.03%.

(4) A high tensile strength steel as described above in any of (1)-(3) further containing, in mass percent, Mo: 0.1-0.8%.

(5) A high tensile strength steel as described above in any of (1)-(4) further containing, in mass percent, at least one of Cr: 0.03-0.80% and B: 0.0002-0.0020%.

(6) A high tensile strength steel as described above in any of (1)-(5) further containing, in mass percent, V: 0.001-0.05%.

(7) A high tensile strength steel as described above in any of (1)-(6) further containing, in mass percent, at least one of Ca: 0.0005-0.005%, Mg: 0.0001-0.005%, and REM: 0.0001-0.01%.

(8) A high tensile strength steel as described above in any of (1)-(7) wherein the value of Pcm given by the following equation (I) is at most 0.25, and for Cu particles dispersed in the steel having a major axis measuring at least 1 nm, they have an average equivalent circle diameter in the range of 4-25 nm and a plane-converted area fraction in the range of 3-20%.

Pcm=C+(Si/30)+(Mn/20)+(Cu/20)+(Ni/60)+(Cr/20)+(Mo/15)+(V/10)+5B  (I)

(9) A marine structure using a high tensile strength steel as described above in any of (1)-(8).

According to the present invention, it is possible to manufacture a high tensile strength steel having excellent weldability and a yield stress of at least 420 N/mm² which can be welded with a welding heat input of at least 300 kJ/cm by a welding method such as electrogas arc welding, although it is not particularly limited to this application. As a result, the efficiency and safety of on-site welding are enormously increased. In addition, it is possible to provide a high tensile strength steel which can be used in extremely severe environments such as in marine structures.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will next be explained in detail. First, the reasons why the present invention limits a steel composition as described above will be explained. In this description, percent with respect to a steel composition refers to mass percent.

C is added in order to attain the strength of steel as well as to produce the effect of refining the structure when Nb, V, and the like are added. If it is less than 0.01%, these effects are not adequate. However, if the amount of C is too large, a hardened structure referred to as island martensite (abbreviated as M-A=martensite-austenite constituent) forms in welds and worsens the toughness of HAZ and has a harmful effect on the toughness and weldability of the base metal. Accordingly, C is at most 0.10%. Preferably it is 0.02-0.08% and more preferably it is 0.02-0.05%.

Si is an element which is effective at preliminary deoxidation of molten steel, but since it does not dissolve in cementite, if a large amount of Si is added, untransformed austenite grains are prevented from breaking down into ferrite grains and cementite, and the formation of island martensite is promoted. For these reasons, the content of Si in the steel is at most 0.5%. Preferably it is at most 0.2% and more preferably it is at most 0.15%.

Mn is an element which is necessary for attaining strength, and it is also effective as a deoxidizing agent. However, addition of too much Mn excessively increases hardenability and causes weldability and HAZ toughness to deteriorate. Mn is also an element which is known to promote center segregation, and from the standpoint of suppressing center segregation, its content should not exceed 1.8%. Accordingly, the content of Mn is 0.8-1.8%. Preferably it is 0.9-1.5%.

P is an impurity element which is unavoidably contained in steel. It is an element which segregates at grain boundaries, and hence it causes grain boundary cracks in HAZ. In order to increase the toughness of weld metals and HAZ and to reduce segregation at the center of a slab, the content of P is at most 0.020%. Preferably it is at most 0.015% and more preferably it is at most 0.01%.

When a large amount of S is present, precipitates in the form of MnS which act as starting points for weld cracking are formed. Therefore, the content of S is at most 0.01%. Preferably it is at most 0.008% and more preferably it is at most 0.005%.

Cu has the effect of increasing the strength and toughness of the steel, and its adverse effect on HAZ toughness is small. In particular, it is necessary to add at least 0.8% Cu in order to expect an effect of increasing strength by epsilon (ε)-Cu precipitation at the time of aging. However, as the Cu content increases, susceptibility to weld cracking at high temperatures increases, and welding procedures such as preheating become complicated. Therefore, the Cu content is made at most 1.5%. Preferably it is 0.9-1.1%.

Ni is an element which effectively increases the strength and toughness of steel and which is also effective at increasing HAZ toughness. However, if it is 0.2% or less, these effects are not obtained, while if it exceeds 1.5%, an effect commensurate with the cost increase is not obtained. Therefore, the content of Ni is 0.2-1.5%. Preferably it is 0.4-1.2%.

Al is an element which is necessary for deoxidation. However, as its content increases, it becomes easy for toughness to deteriorate, particularly in HAZ. This is thought to be because coarse cluster-shaped alumina-based inclusion particles are easily formed. Therefore, the content of Al is 0.001-0.05%. Preferably it is 0.001-0.03%. More preferably it is 0.001-0.015%.

N contributes to refining structure by forming nitrides, but when too much N is added, it causes toughness to deteriorate due to aggregation of nitrides. Accordingly, the content of N is 0.003-0.008%. Preferably it is 0.0035-0.0065%.

By controlling the N/Al ratio to 0.3-3.0, it is possible to increase the toughness of high heat input HAZ and particularly the CTOD properties of weld joints.

This is thought to be because if the N/Al ratio is smaller than 0.3, coarse AlN precipitates. As a result, not only does this have an adverse effect on toughness, but also it impedes dispersion of TiN in fine forms and a large amount. On the other hand, when the N/Al ratio exceeds 3.0, the amount of solid solution N increases, HAZ toughness deteriorates, and the density of dispersed AlN and TiN becomes low. In order to attain better results, a preferred range of the N/AI ratio is 0.4-2.5.

O (oxygen) is effective at forming oxides which become nuclei for the formation of ferrite. If it is present in a large amount, steel cleanliness markedly deteriorates, and it becomes difficult to attain a practical level of toughness in the base metal, the weld metal, and HAZ. Accordingly, the content of 0 is 0.0005-0.0035%. Preferably it is 0.0008-0.0018%.

The following elements are optional elements which can be added to the steel if desired.

Ti forms nitrides and has the effect of suppressing coarsening of crystal grains and refining the structure formed by transformation. However, these effects are not exhibited with a small amount of Ti, and addition of a large amount of Ti has an adverse effect on the toughness of the base metal and of welds. Accordingly, when Ti is added, the content of Ti is 0.005-0.03%. Preferably it is 0.007-0.015%.

Nb increases the strength and toughness of a base metal by grain refinement and precipitation of carbides. However, addition of too much Nb causes the effect of improving the properties of the base metal saturate and the toughness of HAZ's to markedly reduce. Accordingly, when Nb is added, the content of Nb is 0.003-0.03%. Preferably it is 0.003-0.015%.

Mo is effective at attaining hardenability and at increasing HAZ toughness, but if too much is added, it leads to marked hardening of HAZ and deteriorates toughness. Accordingly, when Mo is added, the content of Mo is 0.1-0.8%. Preferably it is 0.1-0.5%.

Cr is effective at increasing the hardenability of steel and at attaining strength. The effect of improving these properties is not exhibited with addition of Cr in a minute amount, while if too much is added, there is a tendency for it to prevent hardening of weld metals and HAZ and increase susceptibility to weld cracking at low temperatures. Accordingly, when Cr is added, the content of Cr is 0.03-0.80%. Preferably it is 0.05-0.60%.

B has the effect of improving hardenability and increasing strength. If too much thereof is added, the effect of increasing strength saturates, and there is a marked tendency for the toughness of the base metal and HAZ to deteriorate. Accordingly, when B is added, the content of B is 0.0002-0.002%. Preferably it is 0.003-0.0015%.

V forms carbonitrides and has the effect of suppressing coarsening of crystal grains and refining the structure formed by transformation. However, these effects are not exhibited with a small amount of V, and addition of a large amount of V has an adverse effect on the toughness of the base metal and of welds. Accordingly, when V is added, the content of V is 0.001-0.05%. Preferably it is 0.005-0.04%.

Ca, Mg, and REM are elements which form oxides and sulfides which become nuclei for precipitation of intergranular ferrite. In addition, they control the form of sulfides and increase low temperature toughness. In order to obtain these effects of Ca, Mg, and REM, it is necessary to contain at least 0.0005% of Ca or at least 0.0001% of Mg or REM. If the content of Ca exceeds 0.005% or the content of Mg or REM exceed 0.01%, large inclusions or clusters composed mainly of Ca or Mg are formed, thereby deteriorating the cleanliness of steel. Accordingly, when Ca is added, the content of Ca is 0.0005-0.005%, and when Mg or REM is added, the content of Mg or REM is 0.0001-0.01%.

In a preferable steel according to the present invention, the value of Pcm represented by the following equation (I) is at most 0.25, and for Cu particles having a major axis measuring at least 1 nm which are dispersed in the steel, the average equivalent circle diameter of the particles is 4-25 nm and the plane-converted area fraction thereof is 3-20%.

Pcm=C+(Si/30)+(Mn/20)+(Cu/20)+(Ni/60)+(Cr/20)+(Mo/15)+(V/10)+5B  (I)

Pcm (weld cracking parameter) is indicative of the susceptibility to weld cracking. If its value is 0.25 or less, weld cracking does not occur under usual welding conditions. Accordingly, Porn is at most 0.25. If Pcm is decreased, it is possible to omit preheating at the time of welding. Preferably it is at most 0.22 and more preferably it is at most 0.20.

The average equivalent circle diameter and the plane-converted area fraction of precipitated Cu particles are now described. The reason why Cu particles having a major axis measuring at least 1 nm are of interest is because Cu particles of smaller than 1 nm contribute little to increasing strength. There are no particular upper limits on the length of the major axis of Cu particles, but when the average value thereof is in the range of 4-25 nm, no particles have a major axis exceeding 100 nm. Cu particles precipitate generally in the form of spheres, but measuring a solid shape is not easy, so projected shapes of Cu particles are measured.

The term “equivalent circle diameter” used herein is the diameter of a circle having an area which is the same as the projected area of a particle. Specifically, it is found from the equation

d=−√{square root over ( )}(4a/π)

wherein a is the projected area (nm²), d is the equivalent circle diameter (nm), and π is 3.14.

For the plane-converted area fraction of Cu particles, a steel material is worked to form a membrane, a portion of the membrane having a thickness of approximately 0.2 micrometers is observed with a TEM, and it is calculated by measuring the percent area occupied by Cu particles when Cu particles which are three-dimensionally distributed in the membrane test piece are projected onto a plane using TEM photograph at a magnification of 100,000.

The reasons why the equivalent circle diameter and the plane-converted area fraction of Cu particles are prescribed in the above manner will be further described.

A characteristic of steel used in marine structures is that in many cases it is an extremely thick high tensile strength steel with a maximum thickness close to 100 mm such that it can withstand external forces applied by waves in a storm. In the future, it will be used for severe conditions, so it will need to satisfy an even more severe CTOD value.

If the strength becomes too high due to Cu precipitation, the CTOD value decreases, and if Cu precipitation becomes inadequate, the strength becomes inadequate even if the CTOD value is high.

There were almost no attempts of a conventional Cu-containing steel being applied to marine structures. Therefore, such steel was not required to satisfy a severe CTOD value, and there was no need to strictly control the average diameter or area fraction of precipitated Cu particles.

In a preferred embodiment of the present invention, in order to achieve a balance between an increase in strength due to Cu precipitation and a decrease in CTOD value, the average diameter and the area fraction of precipitated Cu particles is prescribed in the above manner.

The average equivalent circle diameter of Cu particles is 4-25 nm in order to achieve a balance between strength and toughness, and the plane-converted area fraction thereof is 3-20% also in order to achieve a balance between strength and toughness.

The following are conceivable as factors which can be used for controlling the average diameter and area fraction of Cu particles.

(1) The larger the amount of added Cu, the higher is the area fraction of Cu particles. As for its effect on particle diameter, if the amount of Cu which is added is in a suitable range, the average particle diameter is determined primarily by the structure prior to aging and the temperature and duration of aging. If the added amount is smaller than a suitable amount, the growth of precipitated Cu particles is inadequate and the particle diameter becomes small, while if it is large, there is a tendency for the particle diameter to become large.

(2) The structure before aging has a large influence. The structure before aging is preferably a fine structure comprised primarily of ferrite and bainite.

Since dislocations and particle boundaries become sites of precipitation of Cu particles, a structure including many such precipitation sites facilitates refinement of the Cu particle diameter and increases the area fraction. For this purpose, it is necessary to suitably control the steel composition and make the rolling conditions suitable, and then to select the cooling conditions so as to obtain a fine structure which is comprised primarily of ferrite and bainite.

(3) The temperature and duration of aging are important factors. A desired distribution of Cu particles can be achieved by carefully controlling the diffusion speed of Cu and the growth rate of Cu particles by the aging conditions.

A structure after aging is basically derived from a structure before aging. According to the present invention, a structure after aging comprises at least 70% of a ferrite fraction with the ferritic grain size being 30 micro-meters or less and the flatness of ferrite particles being ⅓ or less.

When the ferrite fraction is at least 70% and the average grain diameter is 30 micro-meters or smaller, stable CTOD characteristics can be obtained at low temperatures.

The flatness of the particles is restricted to 1.5 or less in order to ensure improved resistance to ductile fracture at low temperatures.

The metallurgical structure of a steel plate of the present invention is defined by that in the middle portion of the plate in the thickness direction, i.e., ½t (t: thickness of the plate). The reason why the portion at a depth of ½ of the thickness of the plate is selected is that a metallurgical structure at the middle portion is less sensitive to effects of rolling and cooling after rolling, i.e., the portion can exhibit the worst properties. Especially, when the thickness is as large as 50-100 mm, the mid portion of a plate in the thickness direction is not affected substantially by reduction during rolling and cooling after rolling.

The average diameter of ferrite grains is defined by a diameter of a circle having the same area as that of the grain, i.e., a diameter of a corresponding equivalent circle.

The flatness of the particles is defined by the formula: (grain diameter in the thickness direction of a plate)/(grain diameter in the direction of rolling).

By suitably controlling the above-described three factors while manufacturing the steel according to the present invention, from the above disclosure, a person skilled in the art can carry out the present invention without difficulty.

Next, a method of manufacturing a high tensile strength steel according to the present invention will be explained.

Even with a steel composition as described above, in order to adequately exhibit precipitation hardening by Cu and in order to uniformly provide a high strength and toughness with an increased yield strength at each position in the thickness direction of a thick steel plate with a thickness of at least 50 mm, the manufacturing method must be appropriate.

Steelmaking itself may be carried out by a conventional method and there are no particular restrictions thereon in the present invention. After steelmaking, a steel slab is obtained. From a standpoint of decreasing costs, it is preferable to prepare a slab by continuous casting.

The conditions for heating the slab, hot rolling it, cooling, and tempering will be explained. First, a steel slab having the above-described composition is heated to 900-950° C., and hot rolling is then carried out. In the present invention, in order to obtain a high toughness, it is necessary for austenite grains to be refined sufficiently so as to allow for the formation of the upper bainite structure at the center of the thickness of the resulting thick steel plate. Therefore, in the heating stage, it is important to refine austenite grains inside the thickness of the steel slab. Heating at a temperature lower than 900° C., the formation of solid solution is not adequate, and sufficient precipitation hardening cannot be expected in the tempering step. However, at a heating temperature exceeding 950° C., austenite grains prior to rolling cannot be maintained in a fine and uniform state, and in subsequent rolling, the austenite grains are not uniformly refined. Accordingly, the heating temperature of the steel slab is 900-950° C.

Preferably rolling is performed such that the overall rolling reduction at 900° C. or below is at least 50%. After hot rolling, quench hardening is conducted by water cooling which is commenced at a temperature of at least the Ar₁ point of the steel and terminated at a temperature of 600° C. or below. This is in order to achieve refinement of the structure and to suppress precipitation of Cu particles as much as possible in the stages before aging. If water cooling is commenced at a temperature lower than the Ar₁ point or if cooling is carried out by air cooling, work strains are eliminated, and this causes a decrease in strength and toughness.

The finishing temperature of the hot rolling is preferably at least 700° C., the temperature at the start of cooling is preferably 680-750° C., and the cooling speed up to the temperature at which water cooling is terminated is preferably 1-50° C. per second. If the temperature at the end of water cooling exceeds 600° C., the precipitation strengthening effect in the tempering stage becomes inadequate.

The Ar₁ point is found by a method in which the change in volume of a minute test piece is measured.

After hot rolling and subsequent water cooling, the resulting steel plate is then subjected to aging at a temperature of at least 540° C. and at most the Ac₁ point of the steel, after heating, if necessary, followed by cooling.

The average heating speed up to the aging temperature minus 100° C. in the case where heating is carried out to elevate the temperature to the aging temperature, and the average cooling speed up to 500° C. are controlled. This aging is performed in order to achieve adequate precipitation hardening by precipitated Cu particles, and control of heating/cooling speeds is carried out in order to obtain uniform dispersion of Cu particles. In this respect, heating is preferably performed at an average heating speed of 5-50° C. per minute up to the target aging temperature minus 100° C. with a temperature holding time of at least 1 hour, and cooling is preferably carried out at an average cooling speed of at least 5-60° C. per minute up to 500° C.

In the present specification, the heating temperature is the temperature of the atmosphere inside the furnace used for heating, and the temperature holding time after heating is the length of time for which the steel is kept in the atmosphere inside the furnace. The finishing temperature of hot rolling and the temperatures at the start and completion of water cooling are the surface temperatures of the steel, and the average heating and cooling speeds at the time of reheating are calculated from the temperature calculation at a position at one-half the thickness t of the steel.

In order to construct a large marine structure from a high tensile strength steel according to the present invention, steel materials such as plates, pipes, and shapes are assembled by welding, but in general the steel is used in the form of steel plates.

In the present specification, “excellent weldability” normally means that arc welding with a weld heat input of at least 300 kJ/cm is possible, but other welding methods may also be used such as submerged arc welding and shielded metal arc welding.

Marine structures include not only platforms and jackup rigs which are installed on the sea floor but also semisubmersible rigs (semisubmersible oil drilling rigs) and the like. As long as it is a marine structure requiring weldability and low temperature toughness, there are no particular restrictions. The term “large” structure means that steel used therein has a thickness of at least 50 mm.

EXAMPLES

In this example, steel slabs having a thickness of 300 mm and having the chemical compositions shown in Table 1 and Table 2 were prepared by continuous casting. In order to control inclusions at the center of the thickness of a steel plate, during continuous casting, the temperature of molten steel was not made too high, and the difference thereof from the solidification temperature which was determined by the molten steel composition was controlled so as to be at most 50° C., and electromagnetic stirring just before solidification and reduction in thickness at the time of solidification were carried out.

Table 3 and Table 4 show the working conditions of steel slabs having the chemical compositions shown in Table 1 and Table 2. The working conditions shown in Table 3 and Table 4 are for the steel slabs having the chemical compositions shown in Table 1 and Table 2, respectively.

After a slab with a thickness of 300 mm was heated at the indicated heating temperature for the indicated period, it was subjected to hot rolling and then cooled at an average cooling speed of 5° C. per second by water cooling from the starting temperature to the ending temperature of water cooling. The resulting steel plate had a thickness of 77 mm. (These conditions are shown as the initial heating and rolling conditions in Table 3 and Table 4.)

Reheating was then carried out to the indicated aging temperature, and the temperature was held for the indicated duration (holding time). The heating speed was controlled such that the average heating speed up to the aging temperature minus 100° C. was 10° C. per minute, and the cooling speed was controlled so as to attain an average cooling speed of 10° C. per minute up to 500° C. (These conditions are shown as the aging treatment conditions in Table 3 and Table 4.)

For each of the resulting steel plate, a tensile test piece was taken for a tensile test in accordance in ASTM standards from the center of a plate thickness so that a tensile test piece having a parallel portion with a diameter of 12.5 mm was perpendicular to the rolling direction.

Similarly, a CTOD test of the resulting steel plate was carried out at −40° C. in accordance with BS7448 standards using a 3-point bending test piece which had the full thickness of the plate and which was taken in a direction perpendicular to the rolling direction.

A welded joint was obtained by performing FCAW (flux cored arc welding) at 10.0 kJ/cm on the butt portions of steel plates prepared so as to form a K-shaped groove in accordance with BS7448 standards. From the joint obtained in this manner, a CTOD test piece was obtained by working so as to form a fatigue notch of the CTOD test piece in alignment with the weld line on the straight side of the V-shaped edge portion of the joint, and it was subjected to a CTOD test at −40° C.

In order to ascertain the responsiveness to large heat input welding, the same steel plates having a 20°-shaped beveled edge were abutted and a welded joint was prepared by electrogas arc welding (EGW) with a heat input of 350 kJ/cm. A CTOD test was carried out on the welded joint which was prepared in this manner in accordance with ASTM E1290. The CTOD test piece was obtained by working so as to form a fatigue notch in alignment with the weld line of the joint, and the critical CTOD value was measured at a test temperature of −10° C.

The average value of the equivalent circle diameter of Cu particles was calculated by observing a transmission electron microscope (TEM) photograph at a magnification of 100,000 so as to measure the equivalent circle diameter for each precipitate having a major axis of at least 1 nm and finding the average value of this diameter in each field of view. In order to decrease the variation of measurement, ten fields of view in a TEM photograph (each field of view was a rectangle measured 900×700 nm) taken at a position of one quarter of the initial thickness of the steel material were observed, and the average value was used. In addition, a microphotograph was taken of each of samples of No. 2, No. 8, and No. 36 of Table 1 and No. 61 of Table 2. Namely, a sample was cut from an area including the ½t portion. The sample was embedded in a resin and was ground and buffed to give a minor surface. After etching the surface, the surface was investigated using a microscope at a magnification of 500 times. Image analysis of each of 20 fields (900×700 nm) of microphotography was carried out for each of these samples so as to determine the ferrite fraction ratio and the like.

Table 1 shows test materials satisfying the chemical composition in the present invention. As shown in Table 5, each of these test steels which was manufactured under the working conditions shown in Table 3 had a state of dispersion of Cu particles satisfying a prescribed range.

Regarding microstructures, for No. 2 steel, the ferrite fraction is 75%, the average grain diameter is 24 micro-meters, and the particle flatness is 1.3.

For No. 8 steel, the ferrite fraction is 78%, the average grain diameter is 26 micro-meters, and the particle flatness is 1.2.

For No. 36 steel, the ferrite fraction is 80%, the average grain diameter is 23 micro-meters, and the particle flatness is 1.1.

It is noted from the above that since the billet was heated to 950° C. or less, growth of the crystal grains thereof did not occur so much that the average diameter was kept under 30 micro-meters.

Therefore, the base metal strength, the base metal CTOD properties, and the joint CTOD properties (−40° C. and −10° C.) were high values for each test steel.

In Table 2, No. 40 is a test material which has satisfies the chemical composition and Pcm in the present invention, while Nos. 41-61 are test materials for which the range of chemical composition is outside the range in the present invention. When these test steels were manufactured under the working conditions shown in Table 4, the state of dispersion of Cu particles shown in Table 6 was obtained.

No. 40 satisfied the chemical composition prescribed in the present invention, but the state of dispersion of Cu particles did not satisfy the prescribed range, so the base metal strength was a low value. Accordingly, in order to satisfy both high heat input welding properties and base metal strength, it is desirable to satisfy the condition for dispersion of Cu particles according to the present invention.

Nos. 41-61 did not satisfy the chemical composition in the present invention, and they could not simultaneously satisfy the base metal strength, the base metal CTOD properties, and the joint CTOD properties (−40° C. and −10° C.). According to the present invention, all of these properties must be satisfied simultaneously.

The microstructure of No. 61 test steel was observed. It was noted that the ferrite fraction ratio thereof was 50%, the average grain diameter was 54 micro-meters, and the flatness of particles was 1.8. This microstructure was obtained when the heating temperature of billets was increased to 1160° C. Such a high heating temperature promoted growth of crystal grains. Thus, the ferrite fraction ratio and the flatness of particles of No. 61 test steel are outside the range of the present invention.

Although the present invention has been described with respect to preferred embodiments, they are mere illustrative and not intended to limit the present invention. It should be understood by those skilled in the art that various modifications of the embodiments described above can be made without departing from the scope of the present invention as set forth in the claims.

TABLE 1 Steel C Si Mn P S Cu Ni Mb Mo Al N Ti 1 0.040 0.11 1.45 0.004 0.004 0.93 0.49 0.012 0.19 0.003 0.0041 0.012 2 0.039 0.07 0.87 0.008 0.003 1.01 1.46 0.005 0.37 0.010 0.0047 0.012 3 0.035 0.11 0.98 0.006 0.003 0.96 1.21 — 0.45 0.013 0.0060 0.015 4 0.031 0.09 1.11 0.008 0.004 1.02 1.32 — 0.35 0.005 0.0053 0.012 5 0.031 0.12 1.21 0.004 0.004 0.95 1.41 0.005 0.45 0.003 0.0052 0.015 6 0.030 0.12 1.09 0.004 0.004 0.95 0.95 — 0.42 0.003 0.0052 — 7 0.023 0.12 1.09 0.004 0.004 0.95 0.95 0.003 0.42 0.003 0.0052 0.015 8 0.023 0.12 1.09 0.004 0.004 0.95 0.95 0.003 0.42 0.003 0.0052 0.015 9 0.041 0.15 1.20 0.008 0.002 0.98 1.20 — — 0.004 0.0060 — 10 0.045 0.11 1.10 0.008 0.001 0.97 1.10 0.008 — 0.005 0.0055 — 11 0.039 0.10 1.31 0.009 0.002 0.98 0.87 — 0.32 0.007 0.0056 — 12 0.045 0.09 1.12 0.008 0.001 0.95 1.09 — — 0.008 0.0062 0.011 13 0.038 0.10 1.25 0.009 0.002 0.94 0.63 0.007 0.41 0.007 0.0057 0.007 14 0.037 0.06 1.05 0.009 0.003 0.95 0.52 0.005 0.45 0.003 0.0063 0.008 15 0.035 0.09 1.31 0.009 0.002 0.97 0.59 0.012 0.39 0.006 0.0051 0.007 16 0.041 0.10 1.21 0.009 0.002 0.95 0.56 0.010 0.42 0.005 0.0049 0.009 17 0.043 0.14 1.17 0.009 0.002 1.05 0.42 0.015 0.30 0.007 0.0052 0.008 18 0.042 0.11 1.25 0.008 0.002 0.98 0.48 0.008 0.26 0.006 0.0061 0.011 19 0.037 0.12 1.24 0.008 0.002 0.96 0.64 0.009 0.31 0.009 0.0061 0.010 20 0.041 0.12 1.22 0.008 0.002 0.97 0.71 0.010 0.45 0.008 0.0059 0.012 21 0.033 0.06 1.12 0.006 0.004 0.93 0.81 0.010 0.20 0.009 0.0060 0.013 22 0.047 0.04 1.31 0.006 0.003 1.00 0.51 0.015 0.26 0.004 0.0052 0.009 23 0.049 0.08 1.23 0.009 0.002 0.96 0.62 0.012 0.41 0.009 0.0054 0.009 24 0.048 0.09 0.85 0.009 0.002 0.98 0.94 0.009 0.46 0.009 0.0061 0.010 25 0.041 0.11 1.17 0.009 0.004 1.00 0.88 0.005 0.26 0.009 0.0041 0.010 26 0.042 0.10 1.15 0.008 0.002 0.97 0.84 0.007 0.36 0.007 0.0048 0.011 27 0.041 0.10 1.20 0.008 0.002 0.97 0.67 0.009 0.36 0.008 0.0046 0.009 28 0.031 0.06 1.06 0.010 0.003 0.95 1.00 0.013 0.18 0.006 0.0063 0.012 29 0.044 0.04 1.17 0.005 0.002 0.90 0.46 0.005 0.26 0.003 0.0040 0.008 30 0.049 0.04 0.92 0.005 0.002 0.93 0.62 0.005 0.42 0.009 0.0037 0.010 31 0.036 0.11 1.23 0.009 0.003 1.00 0.64 0.008 0.26 0.007 0.0063 0.008 32 0.041 0.11 1.28 0.005 0.002 0.90 0.43 0.008 0.26 0.006 0.0057 0.009 33 0.041 0.08 0.91 0.008 0.002 1.01 1.18 0.005 0.37 0.010 0.0050 0.010 34 0.038 0.10 0.98 0.008 0.002 0.96 0.87 — 0.45 0.009 0.0060 0.009 35 0.030 0.10 1.02 0.008 0.002 1.02 0.67 — 0.35 0.006 0.0051 0.012 36 0.032 0.11 1.20 0.007 0.002 0.95 1.00 0.005 0.45 0.004 0.0052 0.011 Steel O N/Al Cr V B Ca Mg REM Pcm 1 0.0013 1.3667 0.09 — — — — — 0.188 2 0.0015 0.4700 0.12 — 0.0003 — — — 0.192 3 0.0010 0.4615 0.10 0.005 — 0.002 — — 0.191 4 0.0016 1.0600 0.21 0.005 — — 0.002 — 0.197 5 0.0013 1.7333 — — — — — 0.002 0.197 6 0.0013 1.7333 0.31 — — — — — 0.195 7 0.0013 1.7333 0.31 — — — — — 0.188 8 0.0013 1.7333 0.31 — — — — — 0.188 9 0.0009 1.5000 — — — — — — 0.175 10 0.0008 1.1000 — — — — — — 0.171 11 0.0011 0.8000 — — — — — — 0.193 12 0.0010 0.7750 — — — — — — 0.170 13 0.0010 0.8143 — 0.012 — — — — 0.190 14 0.0011 2.1000 — — 0.0005 — — — 0.180 15 0.0009 0.8500 — — — 0.002 — — 0.188 16 0.0009 0.9800 — — — — 0.0025 — 0.190 17 0.0013 0.7429 0.20 0.008 — — — — 0.196 18 0.0010 1.0167 — 0.011 0.0005 — — — 0.186 19 0.0011 0.6778 0.10 — 0.0006 — — — 0.190 20 0.0010 0.7375 0.15 0.016 0.0007 — — — 0.209 21 0.0014 0.6667 0.31 — — 0.002 — — 0.180 22 0.0016 1.3000 — 0.035 — — — 0.0016 0.193 23 0.0012 0.6000 — — 0.0008 0.003 — — 0.203 24 0.0011 0.6778 0.20 0.022 — 0.002 — — 0.201 25 0.0016 0.4556 — 0.015 0.0012 0.001 — — 0.193 26 0.0013 0.6857 0.21 — 0.0015 0.002 — — 0.207 27 0.0009 0.5750 0.19 0.010 0.0010 0.001 — — 0.204 28 0.0016 1.0500 0.10 — — — — — 0.167 29 0.0014 1.3333 — — — — — — 0.174 30 0.0010 0.4111 0.10 — — — — — 0.186 31 0.0013 0.9000 0.20 — — — — — 0.189 32 0.0018 0.9500 — — — — 0.0016 0.0025 0.178 33 0.0010 0.5000 0.12 — 0.0003 — — 0 0.192 34 0.0012 0.6667 0.10 0.005 — 0.002 — — 0.188 35 0.0013 0.8500 0.21 0.005 — — 0.002 — 0.181 36 0.0013 1.3000 — — — — — 0.002 0.190

TABLE 2 Steel C Si Mn P S Cu Ni Nb Mo Al N Ti 40 0.023 0.12 1.09 0.004 0.004 0.95 0.95 0.003 0.42 0.003 0.0052 0.015 41 0.110 0.15 1.01 0.007 0.005 0.97 0.89 — 0.30 0.012 0.0049 0.010 42 0.061 0.05 1.94 0.005 0.003 0.94 0.95 — 0.20 0.003 0.0052 — 43 0.052 0.08 1.21 0.008 0.002 0.98 1.21 — — 0.019 0.0006 — 44 0.041 0.16 1.05 0.009 0.003 0.95 1.03 0.019 0.26 0.005 0.0055 0.017 45 0.029 0.21 1.24 0.021 0.006 0.47 0.52 0.012 — 0.031 0.0071 0.015 46 0.042 0.35 1.02 0.009 0.006 0.87 0.86 — 0.30 0.112 0.0065 — 47 0.110 0.06 1.28 0.010 0.002 0.93 0.88 0.008 0.50 0.006 0.0041 0.013 48 0.033 0.04 1.39 0.008 0.002 1.94 0.64 0.013 0.42 0.009 0.0052 0.012 49 0.043 0.09 1.39 0.010 0.002 0.98 0.10 0.005 0.42 0.003 0.0063 0.008 50 0.030 0.11 1.39 0.009 0.003 0.90 1.00 0.005 0.34 0.003 0.0041 0.011 51 0.041 0.14 1.06 0.005 0.003 0.93 0.88 0.015 0.85 0.004 0.0046 0.008 52 0.030 0.06 1.39 0.010 0.002 0.90 0.88 0.013 0.18 0.009 0.0057 0.009 53 0.030 0.06 1.50 0.008 0.003 0.90 0.64 0.052 0.42 0.003 0.0063 0.009 54 0.040 0.14 1.06 0.005 0.003 1.00 0.76 0.005 0.42 0.007 0.0041 0.034 55 0.030 0.09 1.17 0.009 0.002 0.90 0.52 0.015 0.26 0.003 0.0052 0.008 56 0.047 0.04 1.17 0.008 0.002 0.90 0.52 0.013 0.18 0.062 0.0032 0.012 57 0.040 0.04 1.39 0.009 0.002 0.90 0.64 0.008 0.50 0.007 0.0110 0.013 58 0.035 0.11 1.06 0.008 0.002 0.90 0.52 0.013 0.26 0.006 0.0046 0.008 59 0.051 0.04 1.17 0.005 0.002 0.90 0.52 0.005 0.26 0.034 0.0031 0.012 60 0.033 0.06 1.39 0.009 0.003 0.95 0.52 0.005 0.42 0.002 0.0078 0.008 61 0.040 0.11 1.45 0.004 0.004 0.95 0.49 0.012 0.19 0.003 0.0041 0.012 Steel O N/Al Cr V B Ca Mg REM Pcm 40 0.0013 1.7333 0.31 — — — — — 0.188 41 0.0015 0.4083 — — — — — — 0.249 42 0.0013 1.7333 0.15 — — — — — 0.243 43 0.0015 0.0316 0.2 0.01 — — — — 0.195 44 0.0051 1.1000 — — — — — — 0.181 45 0.0071 0.2290 — — — — — — 0.130 46 0.0065 0.0580 0.2 — — — — — 0.193 47 0.0016 0.6833 — 0.035 — — 0.0021 — 0.274 48 0.0017 0.5778 0.22 — 0.0009 — — — 0.255 49 0.0017 2.1000 0.40 — — — — — 0.214 50 0.0014 1.3667 0.85 0.008 0.0012 — 0.0025 — 0.237 51 0.0017 1.1500 — 0.035 — 0.0007 — — 0.220 52 0.0016 0.6333 — 0.061 0.0010 — — — 0.184 53 0.0017 2.1000 — — — 0.0012 — — 0.191 54 0.0016 0.5857 — 0.035 0.0021 — — — 0.202 55 0.0014 1.7333 0.16 — 0.0045 — — 0.0021 0.193 56 0.0018 0.0516 — — 0.0026 — — — 0.186 57 0.0017 1.5714 — — 0.0012 — 0.0007 — 0.206 58 0.0041 0.7667 — 0.015 0.0023 — — — 0.176 59 0.0014 0.0912 — — — — — — 0.182 60 0.0013 3.9000 — 0.015 — — — — 0.190 61 0.0013 1.3667 0.09 — — — — — 0.185

TABLE 3 Initial Heating and Rolling Conditions Water cooling Aging Condition Heating Heating Finishing started ended Aging Dura- Steel temp. period temp. at at temp. tion No. (° C.) (hour) (° C.) (° C.) (° C.) (° C.) (hour) 1 950 10 730 710 350 650 5 2 900 10 735 705 350 580 5 3 950 10 750 720 350 575 5 4 900 10 730 720 350 590 5 5 900 10 720 710 350 590 5 6 900 10 730 710 350 650 5 7 900 10 720 710 350 590 5 8 900 10 730 710 350 650 5 9 950 5 730 710 250 600 5 10 950 5 730 710 250 600 5 11 950 5 730 710 250 600 5 12 900 5 730 710 250 600 5 13 900 5 730 710 250 600 5 14 950 5 720 700 250 600 5 15 950 5 730 710 250 600 5 16 950 5 730 710 250 600 5 17 950 5 730 710 250 590 5 18 900 5 710 690 250 600 5 19 950 5 740 720 250 600 5 20 950 5 730 710 250 590 5 21 950 5 730 710 250 630 5 22 950 5 720 700 250 650 5 23 950 5 730 710 250 600 5 24 950 5 740 720 250 600 5 25 900 5 730 710 250 600 5 26 900 5 730 710 250 600 5 27 900 5 720 700 250 600 5 28 900 5 730 710 250 600 5 29 950 5 730 710 250 600 5 30 950 5 730 710 250 600 5 31 950 5 730 710 250 600 5 32 950 5 730 710 250 600 5 33 900 10 735 705 250 580 5 34 950 10 750 720 250 575 5 35 900 10 730 720 250 590 5 36 900 10 720 710 250 590 5

TABLE 4 Initial Heating and Rolling Conditions Water cooling Aging Condition Heating Heating Finishing started ended Aging Dura- Steel temp. period temp. at at temp. tion No. (° C.) (hour) (° C.) (° C.) (° C.) (° C.) (hour) 40 950 10 730 720 350 720 5 41 950 10 750 705 350 600 5 42 950 10 740 710 350 580 5 43 950 10 715 710 350 550 5 44 950 10 720 720 350 620 5 45 950 10 725 710 350 550 5 46 950 10 730 715 350 590 5 47 1000 5 730 720 250 600 5 48 950 5 730 710 250 580 5 49 1000 5 730 720 250 600 5 50 1000 5 720 700 250 600 5 51 950 5 730 710 250 600 5 52 950 5 730 720 250 590 5 53 950 5 730 700 250 590 5 54 1000 5 730 700 250 600 5 55 1000 5 720 700 250 600 5 56 1000 5 730 700 250 600 5 57 950 5 730 700 250 600 5 58 1000 5 730 710 250 600 5 59 950 5 730 700 250 600 5 60 950 5 730 700 250 600 5 61 1160 8 780 760 350 600 5

TABLE 5 Cu particles CTOD of joint Average equivalent Plane-converted CTOD of base low heat high heat Steel circle diameter area fraction Ys Ts metal −40° C. input −40° C. input −10° C. No. (nm) (%) (N/mm²) (N/mm²) (mm) (mm) (mm) 1 16 5 510 571 >1.3 0.61 0.31 2 14 16 580 627 >1.3 0.98 0.41 3 15 17 589 640 1.10 0.79 0.32 4 15 14 563 637 1.09 0.61 0.40 5 15 15 571 641 >1.3 1.10 0.42 6 16 10 512 572 0.89 0.62 0.29 7 14 16 592 646 >1.3 0.83 0.31 8 17 17 509 578 >1.3 1.12 0.28 9 16 15 506 564 1.20 0.62 0.29 10 15 16 499 552 1.00 0.70 0.31 11 16 15 520 579 >1.3 0.56 0.30 12 14 14 497 552 >1.3 0.80 0.42 13 14 16 521 578 >1.3 0.72 0.42 14 15 16 493 572 1.10 0.37 0.41 15 13 14 523 589 >1.3 0.46 0.31 16 14 15 512 584 >1.3 0.53 0.37 17 12 13 530 600 >1.3 0.26 0.27 18 14 14 524 591 >1.3 0.53 0.32 19 15 17 519 587 1.20 0.41 0.33 20 13 16 541 612 1.30 0.63 0.41 21 17 9 497 558 1.10 0.35 0.29 22 18 8 489 553 >1.3 0.31 0.41 23 13 14 531 603 1.10 0.56 0.36 24 14 13 528 600 1.20 0.71 0.31 25 16 15 512 574 >1.3 0.38 0.29 26 14 16 510 581 >1.3 0.42 0.31 27 15 14 507 572 >1.3 0.61 0.36 28 13 13 482 551 >1.3 0.31 0.29 29 12 14 500 571 >1.3 0.42 0.31 30 13 16 510 581 >1.3 0.35 0.31 31 12 15 502 573 >1.3 0.32 0.36 32 14 14 501 569 >1.3 0.31 0.34 33 14 16 580 627 1.10 0.94 0.40 34 15 17 584 642 0.94 0.79 0.35 35 15 14 560 637 1.09 0.54 0.41 36 15 15 569 640 1.10 0.63 0.42

TABLE 6 Cu particles CTOD of joint Average equivalent Plane-converted CTOD of base low heat high heat Steel circle diameter area fraction Ys Ts metal −40° C. input −40° C. input −10° C. No. (nm) (%) (N/mm²) (N/mm²) (mm) (mm) (mm) 40 28 2 417 472 >1.3 0.67 0.30 41 17 16 626 711 0.12 0.09 0.006 42 14 14 642 720 0.19 0.12 0.006 43 14 13 632 691 0.41 0.06 0.007 44 17 17 578 632 0.08 0.09 0.007 45 10 15 384 420 0.84 0.71 0.006 46 17 15 574 623 0.12 0.04 0.006 47 16 15 621 716 0.10 0.03 0.007 48 18 23 619 701 0.08 0.02 0.007 49 14 16 512 589 0.08 0.03 0.006 50 15 15 623 712 0.08 0.04 0.007 51 14 16 621 700 0.09 0.01 0.008 52 13 14 552 636 0.08 0.02 0.008 53 13 15 541 617 0.09 0.01 0.006 54 17 13 543 675 0.09 0.03 0.008 55 14 17 567 648 0.08 0.04 0.009 56 14 13 511 652 0.04 0.03 0.008 57 14 14 498 612 0.06 0.02 0.008 58 16 15 470 530 0.06 0.03 0.009 59 16 16 502 580 0.21 0.16 0.018 60 15 15 491 565 0.22 0.17 0.019 61 31 15 570 630 0.12 0.53 0.56 

1. A high tensile strength steel consisting essentially, in mass percent, of C: 0.01-0.10%, Si: at most 0.5%, Mn: 0.8-1.8%, P: at most 0.020%, S: at most 0.01%, Cu: 0.8-1.5%, Ni: 0.2-1.5%, Al: 0.001-0.05%, N: 0.003-0.008%, O: 0.0005-0.0035%, Ti: 0-0.03%, Nb: 0-0.03%, Mo: 0-0.8%, Cr: 0-0.80%, B: 0-0.002%, V: 0-0.05%, Ca: 0-0.005%, Mg: 0-0.005%, REM: 0-0.01%, and a reminder of Fe and impurities, wherein the N/Al ratio is 0.3-3.0, wherein the value of Pcm represented by the following equation (I) is at most 0.25, and for Cu particles having a major axis with a length of at least 1 nm dispersed in the steel, the average equivalent circle diameter of the Cu particles is in the range of 4-25 nm and the plane-converted area fraction thereof is in the range of 3-20%. Pcm=C+(Si/30)+(Mn/20)+(Cu/20)+(Ni/60)+(Cr/20)+(Mo/15)+(V/10)+5B  (I)
 2. A high tensile strength steel as set forth in claim 1 which contains, in mass percent, Ti: 0.005-0.03%.
 3. A high tensile strength steel as set forth in claim 1 which contains, in mass percent, Nb: 0.003-0.03%.
 4. A high tensile strength steel as set forth in claim 1 which contains, in mass percent, Mo: 0.1-0.8%.
 5. A high tensile strength steel as set forth in claim 1 which contains, in mass percent, at least one of Cr: 0.03-0.80% and B: 0.0002-0.002%.
 6. A high tensile strength steel as set forth in claim 1 which contains, in mass percent, V: 0.001-0.05%.
 7. A high tensile strength steel as set forth in claim 1 which contains, in mass percent, at least one of Ca: 0.0005-0.005%, Mg: 0.0001-0.01%, and REM: 0.0001-0.01%.
 8. A high tensile strength steel as set forth in claim 1 wherein the steel is manufactured by heating a billet at a temperature of 950° C. or less, hot rolling the heated billet, cooling the resulting hot rolled steel, and tempering the cooled steel.
 9. A steel plate comprising a high tensile strength steel as set forth in claim 1, wherein the ferrite fraction of the plate is at least 70%, the average diameter is 30 micro-meters or less, and the flatness is 1.5 or less at a depth of ½t in the thickness direction of the plate.
 10. A marine structure using a high tensile strength steel as set forth in claim
 1. 